Receiving apparatus for communication systems



w. P. PLACE July 26, 193s.

RECEIMINU APPARATUS FOR COMMUNICATION SYSTEMS Filed May 17,v 1954INVENTOR lfl'llaldR Place.

l' Dmc/126 HIL; ATTORNEY BYQ Patented July 26, 1938 UNITED STATES PATENTOFFiQE RECEIVING APPARATUS FOR COMMUNICA- TION SYSTEMS Application May17, 1934, Serial No. 726,130

8 Claims.

My invention relates to receiving aparatus for communication systems,and particularly to receiving apparatus for telephone communication withvehicles in motion as, for example, a railway train. A

I will describe one form of apparatus embodying my invention, and willthen point out the novel features thereof in claims.

Communication systems have been proposed for telephoning between awayside station and a train, or between two diierent trains, or betweentwo spaced locations on the same train, by transmitting froml onelocation to the other a carrier current modulated with voice frequenciesor else suppressing the carrier and transmitting a side band of suchmodulated current. Such systems preferably utilize the traflic rails asthe communication channel, the current being supplied to the traicvrails at the sending location by coupling the sending transmitter withthe traflic rails. The current ows in the traffic rails in either orboth directions fromthe point at which it is supplied to the rails andreturns through the ground. It has been found that a line wire such, forexample, as a line wire commonly used in telegraph work, through itsdistributed capacity with the earth and with the traflic rails alsoserves as a path for such current. It would be helpful, therefore, to be.able to receive current aboard a train from the line wires parallelingthe traic rails. It is not convenient, however, to do this altogethersince there are places along a railway where there are no line wires'present and where it would not be convenient to install aparallelingline wire. A feature of my present invention is the provision ofreceiving apparatus for communication systems 'of the type referred towherewth signaling current supplied to the traflic rails at one locationwill be effectively received aboard a train either from the trac railsor from line wires paralleling the traffic rails or from both. Theseparate currents picked up from these two channels are independentlyfiltered, amplified and detected and then combined for actuating asignaling device.

The receiving apparatus for railway telephone systems is preferably incircuit at all times ready to receive a message from the remote pointand preferably includes a loudspeaker, with the result that earthcurrents or stray magnetic fields picked up during the non-transmittingperiod of the system produce a noise in the loud-speaker that sometimesbecomes very annoying, especially since high gain ampliers are employed,During the telephoning period such unwanted energy when picked up mayrender the reception of speech unsatisfactory and unintelligible. It hasbeen found that the ratio between signal and noise in the line Wiresparalleling a railway track is often much better than the ratio betweensignal and noise as found in the traic rails. On the other hand, theremay be times when there is more noise in the line wires than in thetrack. A further feature of my invention is the provision of receivingapparatus for such telephone systems that will automatically select thebetter of two channels in accordance with the noise existing on thechannels. That is, if the amount of noise received over one channelduring non-transmitting period is small compared to the amount of noisereceived over the other channel, the sensitivity of the highfrequencyamplierfor the channel having little noise is automaticallyadjusted at relatively high gain While the sensitivity of the highfrequency amplifier for the channel having a large amount of noise isautomatically adjusted at relatively low gain. Then, when conversationis established, the telephone current received over the channel havinglittle noise will be amplified at high gain while the telephone currentreceived over the channel having a large amount of noise will beamplified at relatively low gain and the Lmwanted noise current will beamplied but little. The amplification of the telephone currents received-from the two channels will be also kept down to a level where thecombination of the two after detection will not produce a signal that isobjectionable.

In explorations along railway tracks, I have found that a considerablephase difference between the telephone current picked up from thetraffic rails and that picked up fromthe paralleling line wires mayexist, so that, the `phase of the two currents received aboard a trainmay be such as would cause these currents to cancel each other. A stillfurther feature of my invention is the provision of receiving apparatuswherewith carrier telephone current received over two independentchannels may be combined with little loss in eiectiveness on thesignaling device, notwithstanding a considerable phase differencebetween the two currentsas picked up may exist. Other features andadvantages of my invention will appear as the specification progresses.

A better understanding of my invention may be had from the accompanyingdrawing which is a diagrammatic View of one form of receiving apparatusembodying my invention. In the drawing, 3 and 4 designate inductor coilsmounted on a railway train in inductive relation with the traffic railsI and Ia respectively. This receiver 3-4 may be mounted on any desiredvehicle of the train and will be referred to in the followingdescription as being mounted on the caboose of a freight train in orderto clarify the description. The outside terminals of the receiver 3-4are connected over wires 5 and IiV with the input of a band-pass filterBPF, the two portions or coils 3 and 4 being connected in series andarranged that voltages induced therein g Vsion is employed, the upperside-band being transmitted. Under these conditions the band- Vpassfilter BPF may be any one of the wellknown types for such filters, whichis so proportioned and adjusted as to pass a band of frequenciesextending from 8,500 to 10,500 cycles per second, the voice frequenciesbeing taken as extending from 500 to 2,500 cycles. It follows thatfrequencies within such upper side-band range picked up from the trafficrails will be passed to the input of the high frequency ampliiier withvery little attenuation while frequencies outside of such range will besubstantially suppressed.

The high frequency amplifier HFA may be of any of the several typeswell-known to the art and may consist of one or more stages ofamplification as desired. The type of this amplifier forms no part of myinvention, and it will be described only sufciently to point out itsrelationship with my invention. As here illustrated, this high frequencyamplifier consists of several stages of amplification the final stage ofwhich includes an electron tube 9 while the stages ahead of the finalstage are shown conventionally only at I0. It will be understood thatthese stages ahead of the nal stage each preferably includes an electrontube and aiiiliated circuits 'similar to that about to be described forthe tube 9. The tube 9 is preferably a tetrode having an indirectlyheated cathode II, a plate I2, a screen grid I3 and a control grid I4.The control grid circuit for the tube 9 is inductively coupledwith theoutput circuit of the stage ahead through the medium of a transformerT1, the primary winding 'I4 of which is included in the output circuitof that stage. The secondary winding I5 of the transformer T1 has oneterminal connected with the control grid I4 and its other terminalconnected with the cathode I-I through a biasing unit I6. The controlgrid I4 is also connected with an automatic volume control device to bedescribedhereinafter. The plate I2 of the tube 9 is provided with acircuit that includes the primary winding I'I of a transformer T2 and asource of current here shown as a battery I8 as will be understood by aninspection of the drawing. The screen grid I3 kof the tube 9 isconnected with an intermediate point of the battery I8 over a wire I9,the filament of the tube is heated by a battery 20 and the cathodeelement II is connected with a ground electrode 45. Consequently,side-band frequencies picked up from the traffic rails and passed by thefilter BPF will be successively amplified at the different stages of theamplifier HFA, will be reproduced in the primary winding Il of thetransformer T2 greatly increased in amplitude and will, in turn, inducean electromotive force of corresponding frequencies in the secondarywindings 2I and 22 of the transformer T2.

The electromotive force induced in the secondary winding 2| of thetransformer T2 is applied to the input of a detector or demodulator DM,the output of which is further amplified for operating a signalingdevice. As here shown the opposite terminals of the secondary winding 2|are connected with the input of the demodulator DM over the wires 23 and24, and the output of the demodulator DM is connected with a primarywinding 25 of a transformer T3 over wires Il and 18. The demodulator DMmay be any one of several types as desired, and as its specie type formsno part of my invention it is shown conventionally for the sake ofsimplicity.

'It is deemed suiiicient for this application to say that the upperside-band frequencies extending from 8,500 to 10,500 cycles per secondapplied to theA input of the demodulator are combined with a localcarrier and demodulated causing to appear in its output circuit thevoice frequencies extending from 500 to 2,500 cycles. The manner ofutilizing the voice frequency current supplied to primary winding 25 oftransformer T3 from the demodulator DM will be fully describedheroinafter.

The electromotive force induced in the secondary winding 22 of thetransformer T2 is utiliZed for automatically adjusting the gain of thehigh frequency amplifier through the medium of an automatic volumecontrol device AVC. The opposite terminals of the secondary winding 22are connected with the input terminals of a fullwave rectifier 28included in theautomatie volume control device AVC. The loutputterminals of the rectifier 28 are connected across a capacitance 39 anda resistor 29 in parallel which are also included in the device AVC. Itfollows that that'portion of the amplified side-band passed to thesecondary winding 22 is rectified and the capacitance 30 is charged at apotential substantially equal to the peak value of the voltage inducedin the secondary winding 22, the polarity of the rectier 28 andcapacitance 30 being as indicated by the plus and minus signs located onthe drawing.

The potential build up on the capacitance 30 is applied to the grids ofthe electron tubes` of the different stages of the amplifier' HFA forcontrolling the sensitivity of the respective tubes. As here shown thepositive terminal of the automatic volume control device AVC isconnected with a ground electrode 44 over wire 49, thence through amaster volume control device MVC in a manner to be more fully describedhereinafter and wire I3. Referring to the tube 9 of the nal stage of theamplifier HFA, a grid leak consisting of two resistors 3l and 32 inseries is provided and its terminal remote from the grid I4 is connectedwith the negative terminal of the automatic volume control device AVCover wire 33. A condenser 34 is connected between the mid-terminal ofthe resistors 3| and 32 and the terminal of the grid bias unit I6 remotefrom the cathode II, and a condenser 35 may be inserted between thiscathode connection and the wire I9 leading to the screen grid I3 forstabilizing the circuit. Recalling that the cathode II is Vconnectedwith the ground electrode 45, it is clear that the negative bias voltageof the grid I4 is governed not only by the normal biasing unit I6 but,also by the potential built up on the capacitance 30 of the automaticvolume control device AVC. The negative terminal of the automatic Volumecontrol device AVC is also connected with the control grids of the tubesincluded in the other stages of the amplifier HFA over the wires 33, and36 as indicated on the drawing. Hence, the negative grid bias of eachtube of the different stages of the amplifier HFA is adjusted accordingto the output of the amplifier.

To sum up, when current is applied to the input circuit of the ampliiierHFA and an electromotive force is induced in the secondary winding 22 ofthe transformer T2, this alternating electromotive force is rectifiedand a potential is created on the capacitance 30 which potential isapplied to the grids of the electron tubes of each of the stages of theamplifier in such a manner that the negative potential of the grid withrespect to the cathode is varied. For example, the parts are soproportioned and adjusted that the tube S will be operating atsubstantially maX- imum sensitivity at the normal negative grid bias`and will continue to operate at substantially the same sensitivity untilthe potential applied through the automatic volume control devicebecomes greater than that produced by the desired energy level of thesignaling current. When the output of the amplier increases above thedesired signal level, the negative bias .applied to the grid of the tube9 will rapidly decrease the sensitivity of the tube with the result thatunwanted noise currents having an energy level greater than that of theusual signaling current will be amplified at the tube 9 little or none.The automatic volume control device AVC being also connected with theinput circuit of the stages ahead of the tube 9 as pointed outhereinbefore, the gain at each stage of the amplifier will be adjustedin'a like manner in response to unwanted currents above the desiredlevel.

When the output of the amplifier HFA remains substantially constant, abalance is quickly established and the only power taken by the automaticvolume control device is that substantially negligible amount necessaryto maintain the charge on the capacitance 30. Upon an increase in theoutput of the amplier the charge of the capacitance 30 will becorrespondingly increased and the negative grid bias applied to thetubes of the amplifier will be also increased. The change in theamplification effected by this increase in the negative bias upon thedifferent tubes will depend upon the magnitude of the output and in alesser manner upon the normal grid bias. As there is nothing in thecircuit to cause any appreciable delay the desensitizing, of Whatevermagnitude, will occur relatively rapid. Upon a decrease in the output ofthe amplifier the potential built up on the capacitance 30 will leak offthrough the resistor 29 and a corresponding change will be effected uponthe negative grid bias of the tubes to restore the sensitiveness of theamplifier. The capacitance and resistor of the automatic control deviceare preferably made such that the resto-ring of the sensitiveness of theamplifier will be relatively slow. Consequently, the automatic volumecontrol device will be rapid enough to desensitize the amplifier in theevent of sudden unwanted noise currents and will be slow enough inrestoring the sensitivity of the amplifier that short conversations Willnot be affected thereby. It is to be seen, therefore, that the gain ofthe amplifier HFA will be automatically modified in accordance with itsoutput voltage, which, in turn is applied to the demodulator, and largevoltages due to earth currents or stray magnetic elds will have theireiect upon the demodulator DM suppressed.

31 is an inductor coil mounted on the caboose at any convenient pointand preferably at the top of the vehicle. This inductor is preferablyone of relatively large cross section and placed so that its axis isvertical causing it to be in inductive relation with a line wireparalleling the traffic rails such, for example, as the wire 2. That isto say, the inductor or receiver 31 is responsive to current flowing inthe line wires and to a lesser extent to currents flowing in otherconductors paralleling the traflic rails but will not respond to currentflowing in the traiiic rails on which the train is running. The oppositeterminals of the receiver 37 are connected with the input of a band-passfilter BPF over wires 33 and 39. This band-pass iilter is identical tothe filter associated with the receiver 3--4 and has its outputconnected with the input circuit of a second high frequency ampliiierHFA1 also mounted in the Caboose and which is preferably identical instructure to the high frequency amplifier HFA. The final stage of theamplifier HFA1 includes a tetrode tube 4Q which is provided with plateand grid circuits similar to the circuits described for the tube 9, andit is thought unnecessary to repeat the description. Suiiice it to saythat frequencies of the range of 8,500 to 10,500 cycles picked up by thereceiver 3l and passed by the associated filter BPF will be successivelyamplified at the diiferent stages of the amplifier HFAl and will induceelectromotive forces in the secondary windings 4l and i2 of atransformer T4, the primary winding 43 of which is included in the platecircuit of the tube 40. The electromotive force induced in the secondarywinding 4l of the transformer T4 is passed to the input of a demodulatorDMl preferably similar to the demodulator DM and hence the currentsupplied by the secondary winding di is demodulated and correspondingvoice frequencies are caused to appear in the output of the demodulatorDM1 which includes a primary winding 5l' of the transformer T3, theconnection being over wires 55 and 56.

The electromotive force induced in the secondary winding 42 of thetransformer T4 is applied to the input terminals of a rectifier 4Sincluded in an automatic volume control device AVC1, the outputterminals of the rectiiier 455i being connected across a capacitance 48and a resistor 41 in parallel with the result that the capacitance 48 ischarged at a potential substantially equal to the peak value of theelectromotive force induced in the secondary winding 42. The polarity ofthe output terminals of the rectier 4S and of the opposite plates of thecapacitance 43 is indicated by the plus and minus signs placed on thedrawing. The positive terminal of the device AVC1 is connected with theground electrode 44 over wires 50 and 49 and thence through the mastervolume control device MVC, while the negative terminal is connected overwire 53 with the grid leak of the tube 46 which includes the tworesistors 5l and 52 in series. by proper proportioning of the parts thegain of the amplifier HFAl will be automatically adjusted through theautomatic volume control device AVC1 in the same manner as pointed outhereinbefore in describing the control of the It is clear that 7 deviceAVC over the gain of the amplifier HFA. It` will be Vnoted that thenegative terminal of the device AVC1 will be connected with the gridcircuits of the tubes for the stages ahead of the tube 45 by means ofthe wire 54.

It is to be seen, therefore, that the portion of the telephone currentpicked up from the channel including the traffic rails, and that theportion picked up from the channel including the line Wire 2 areindividually filtered, amplified and demodulated'and the correspondingaudio frequencies are combined at the transformer T3, since the outputsof the two demodulators are supplied to the two primary windings 25 and51 of the transformer T3 each of which will be effective to induce anelectromotive force in the secondary winding 26 of that transformer. Aportion of the output of each of the high frequency amplifiers isutilized through an associated volume control device for adjusting thegain of the respective amplifier.

The audio or low frequency electromotive force induced in the secondarywinding 26 of the transformer T3 as a result of the different currentssupplied to the two primary windings 25 and 51 is utilized for operatinga signaling device, in this instance the current being first furtheramplified at the electron tube 21. The amplifying tube 21 is preferablya pentode having a heated filament 58, a plate 59 and two control grids60 and 6I as well as a cathode screen grid for screening the secondaryemission from the plate. The control grid 60 is provided with a circuitwhich includes the secondary winding 2S of the transformer T3, and theplate 59 is provided with a circuit which includes a battery 62 and theprimary winding 63 of a transformer T5. The grid 6l is connected withthe terminal of the primary winding 63 remote from the plate through aresistor 64 across which is preferably connected two condensers 65 and66 in series with a ground electrode 61 connecting at the mid-terminalof the condensers. It is clear that the electromotive force induced inthe secondary winding 25 of the transformer T3 will be reproduced in theprimary winding 63 of the transformer T5 greatly increased in amplitudewhich will, in turn, induce electromotive forces of like frequencies inthe secondary windings S8 and S9 of the transformer T5. The energysupplied to the secondary winding 68 is delivered to the operatingwinding of a loud-speaker LS and hence the audio frequencies induced inthe secondary winding 26 of the transformer T3 will be reproduced at theloud-speaker LS.

The portion of the output of the audio amplifier 21 supplied to thesecondary winding 69 of the transformer T is applied to the inputterminals of a full-wave rectifier 10 included in the master volumecontrol device MVC, the output terminals of the rectifier 1B beingconnected across a resistor 1l and a capacitance 12 in parallel. Thepolarity of the output terminals of the rectifier 10 is indicated by theplus and minus signs. Consequently, the electromotive force induced inthe secondary Winding 59 is rectified and the capacitance 12 is chargedat a potential substantially equal to the peak value of suchelectromotive force. The positive terminal of the master volume controldevice MVC is connected with the ground electrode 44 over wire 13, whileits negative terminal is connected with the wires 49 and 50 leading tothe positive terminals of the automatic volume control devices AVC andAVCl, respectively, as traced hereinbefore. It is to be noted'that" theconnection ofthe devices AVC and AVC1 with the ground electrode 44includes the different arms of the rectifier 10 in the low resistancedirection of the asymmetric units and the resistor 1l with the resultthat little resistance is interposed by the master volume control devicein the ground connection for the automatic volume control devices AVCand AVC1. It is to be seen thatV the potential established for thecapacitance 12'of the master volume control device MVC will operate inseries with the charge established on the capacitances 30 and 48 of theautomatic volume control devices AVC and AVC1, respectively, to'adjustthe negative grid bias of the respective amplifier, the potential of thecapacitance 12 being effective to vary the grid bias of each of the highfrequency amplifiers HFA and I-IFA1 simultaneously and by equal amounts.That is, a portion of the output of the audio frequency amplifier isutilized for simultaneously adjusting the gain of the two high frequencyamplifiers. The parts of this master volume control device MVC are soproportioned that when the output of the audio amplifier 21 reaches alevel that the response of the loud-speaker becomes too great, the gainof the high frequency amplifiers is reduced and the energy for operatingthe loud-speaker adjusted to a more desirable level.v

In describing the operation of my invention I shall first assume that anon-transmitting period exists. Under this condition, whatever noisecurrent is picked up from the trafiic rails or from the line wires andpassed to the respective high frequency amplifiers, the associatedautomatic volume control devices will be effective to adjust the gainofeach'amplifier accordingly, so that substantially an equal amount ofnoise is applied to each demodulating device. For instance, if theamount of noise received from the channel including the line wires isonly one thousandth of the amount of noise received from the' channelincluding the traffic rails, the high frequency amplifier HFAl will haveits sensitivity automatically adjusted to a relatively high gain whilethe amplifier HFA will have its sensitivity adjusted at a relatively lowgain, and the resultant noise currents applied to the input of thedemodulators DMl and DM will be of substantially equal levels. In otherwords, due to the high level of noise in the trafc rail channel, thechannel including the line wires is automatically made the preferredchannel for receiving a signal by the amplifier HFA1 being 'adjustedA athigh sensitivity and the amplifier HFA being adjusted at lowsensitivity. Should' the noise received from the line wire channel begreat and that received from the traflic rail channel low then the trackchannel is automatically made the preferred channel for receiving thesignal by the amplifier HFA being adjusted at high sensitivity and theamplifier HFA1 adjusted at low sensitivity. It is apparent, therefore,that the more desirable channel for receiving a signal is automaticallyselected during the non-transmitting period.

When a conversation is begun and the carrier current modulated by thevoice frequencies is supplied'to the traffic rails at the sendinglocation, the receiver 3-4 and the receiver 31 will each pick up someelectromotive force corresponding to the telephone current which isbeing transmitted. If the telephone current picked up by the receiver 31is several times as great as the noise it is receiving, satisfactorycommunication can be carried out over the vline wire channelnotwithstanding the noise level of the trafiic rail channel may berelatively high. The energy corresponding to the voice frequenciespicked up from the line wire channel will be filtered, amplified anddemodulated independent of that picked up from the traffic rails. 'I'hetelephone current picked up from the trafic rails will also be filtered,amplified and demodulated but, as we are now assuming that the railchannel is noisy, the rail current will be amplified at low gain withthe result that, when the two currents are combined after demodulation,the portion corresponding to the line wire current will greatlypredominate and the combination will contain a relatively low noiseenergy. After the two audio frequency currents are combined, thecombination will be further amplified and applied to the loud-speaker LSwhere the telephone message will be reproduced. The master volumecontrol device MVC will adjust the sensitivity of both the highfrequency amplifiers so that if the energy level of the combinedtelephone current as applied to the loud-speaker is higher thannecessary for desirable operation their gain will be decreased and thenoise will still further fade into the background. The capacitance 12and resistor 1l of the master volume control device MVC are soproportioned that relatively rapid action in both increasing anddecreasing the sensitivity of the high frequency amplifiers obtains and,hence, the desired amount of energy for desirable operation of theloud-speaker at all times is assured. It is obvious that if the signalto noise ratio as received from the traffic rail channel is better thanthat of the line wire channel then the rail channel will be made thepreferred channel for communication. Furthermore, if both channels arerelatively free from noise and sufficient energy for communication isreceived over each, the master volume control device will operate todesensitize each high frequency amplifier to a point where the voltageof the combined audio frequencies is at a level for desirable operationof the loudn speaker.

It may happen that the telephone voltage received from the two channelswill be equally good but out of phase. Such a condition will notseriously affect the operation of the loud-speaker because thedifference in phase will be a relationship existing between the carrieror side-band frequencies which are materially higher than the audiofrequencies that operate the loud-speaker. Should the telephone voltagespicked up from the two channels be out of phase as much as 180 therewill be no cancellation since each will be individually amplified anddemodulated. After demodulation, addition of the two voice frequencieswill not be seriously affected since the phase relationship of therelatively low voice frequencies compared with the 8000 cycle carrierwill not be over 45 out of phase at most. Thus, if the currents pickedup from the different channels are considerably out of phase, they willstill be near enough in phase to add with little loss at the audiofrequency amplifier where they are combined. That is to say, that byindividually amplifying and demodulating the currents as received fromthe two different channels and then combining the resultant audiofrequency currents phase displacement of the two carrier currents willnot seriously affect the final telephone current as applied to theloud-speaker.

Although I have here described a particular adaptation for receivingapparatus embodying my invention, it will be understood that I do notwish to limit myself to telephone systems for railway trains, andreceiving apparatus embodying my invention is equally useful for othercommunication systems wherein a signal current is received over twopartially independent channels. The application here shown will serve toillustrate the many places apparatus embodying my invention may beemployed.

Although I have herein shown and described only one form of apparatusembodying my invention, it is understood that various changes andmodications may be made therein within the scope of the appended claimsWithout departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

l. Receiving apparatus for communication systems having two channelsover either one or both of which a carrier current modulated with voicefrequencies flows comprising, a first and a second receiver each coupledto a distinctive one of said channels, a first and a second amplifiereach including an electron tube, means for connecting each receiver withthe grid of a distinctive one of said tubes for individually amplifyingthe voltage picked up from said channels, an automatic volume controldevice for each amplifier each including a rectifier having its inputterminals connected with the plate circuit of the associated tube and acapacitance connected across the output terminals of the rectifier andsaid capacitance having its high potential plate grounded and its lowpotential plate connected with the grid of the tube for adjusting thegain of the amplifier according to the magnitude of the voltage pickedup from the associated channel, a rst and a second demodulator, meansfor connecting the input circuit of each demodulator with the outputcircuit of a distinctive one of said amplifiers for individuallydemodulating the carrier telephone voltages received from said channels,a third amplifier including an electron tube, means for coupling thegrid circuit of said last mentioned tube with the output of both of saiddemodulators for amplifying the combined audio frequency currents, aloud-speaker coupled with the plate circuit of said last mentioned tubefor reproducing the audio frequencies, and a third volume control deviceincluding a rectifier having its input terminals connected with theplate circuit of the tube of said third amplifier and a capacitanceconnected across the output terminals of the rectifier andrsaidcapacitance having its high potential plate grounded and its lowpotential plate connected with the gri-ds of the tubes of said first andsecond amplifiers in parallel for adjusting the gain of both of saidfirst and second amplifiers simultaneously in accordance with themagnitude of the combined audio frequency currents.

2. Receiving apparatus for communication systems in which a telephonecarrier current is supplied to a communication channel comprising, areceiver coupled with said channel for picking up such current, a highfrequency amplifier including an electron tube, means for connecting thegrid circuit of said tube with said receiver, a demodulator, means forconnecting the input of the demodulator with the plate circuit of saidtube, an audio frequency amplifier including an electron tube, means forconnecting the grid circuit of said last mentioned tube with the outputof the demodulator, a telephone receiver connected with the platecircuit of said last mentioned tube, a rst volume control deviceincluding a rectifier and a capacitance and said device having its inputterminals connected with the plate circuit of the high frequencyamplifier tube and its output terminals connected with the grid of saidtube for adjusting the sensitivity of the tube according to themagnitude of the current picked up from said channel, and a secondvolume control device including a rectifier and a capacitance and saiddevice having its input terminals connected with the plate circuit ofthe audio frequency amplifier tube and its output terminals connectedwith the grid of the high frequency amplifier tube serially with thefirst volume control device for adjusting the sensitivity of said highfrequency amplifier tube according to the magnitude of the currentsupplied to said telephone receiver.

3. Receiving apparatus for communication systems in which a modulatedcarrier current is supplied to a communication channel comprising, areceiver coupled with said channel` for picking up such current, a highfrequency amplifier including an electron tube, means for connecting thegrid circuit of said tube with said receiver, a demodulator, means forconnecting the input of the demodulator with the plate circuit of saidtube, a low frequency amplifier including an electron tube, means forconnecting the grid circuit of said last mentioned tube with the outputof the demodulator, a signaling device connected with the plate circuitof said last mentioned tube responsive to the modulating frequency ofthe carrier, a first volume 'control device coupled with the platecircuit of the high frequency amplifier tube and having connection withthe grid of said tube for adjusting 'the grid bias voltage of the tubeaccording to thermagnitude of the current supplied to the demodulator,and a second volume control device coupled with the plate circuit of'the low frequency amplifier tube and having connection with the gridofthe high frequency amplifier tube serially with said first volumecontrol device for adjusting the grid bias voltage of that tubeaccording to the magnitude ofthe current supplied to said signalingdevice.

4. Receiving apparatus for communication systems for railway trains inwhich a carrier telephone current flows partly in the traffic rails andpartly in a line wire paralleling said rails comprising, a first and asecond receiver on a train coupled with the rails and with said linewire respectively, first and a second high frequency amplifier eachincluding an electron tube, means for coupling the grid circuit of eachtube with a particular one of said receivers, a first and a seconddemodulator, means for coupling the input circuit of each demodulatorwith the plate circuit of the tube of a particular one of saidamplifiers, an audio frequency amplifier including an electron tube,means for coupling the grid circuit of an audio frequency amplifier tubewith the output circuits of both of said demodulators, a loud-speakercoupled with the plate circuit of said audio amplifier tube, a first anda second volume control device each including a rectifier, means forcoupling the input terminals of each rectifier with the plate circuit ofa particular one of the high frequency amplifiers and the outputterminals of each rectifier with the grid circuit of the same highfrequency amplifier tube, a master volume control device including arectifier, and means for coupling the input terminals of the lastmentioned rectifier-with the plate circuit of the audio frequencyamplifier tube and its output terminals with the grid circuits of `thetwohigh frequency amplifier tubes in parallel.

5. Receiving apparatus for communication systems for railway trains inwhich a carrier telephone current fiows partly in the traffic rails andpartly in a line wire paralleling said rails comprising, a first and asecond receiver on a train coupled withthe rails and with said line wirerespectively, a first and a second high frequency amplifier eachincluding an electron tube, means for coupling the grid circuit of eachtube with a particular one of said receivers, a first and a secon-ddemodulator, means for coupling the input circuit of each demodulatorwith the plate circuit of a particular one of said amplifiers, an audiofrequency amplifier including an electron tube, means for coupling thegrid circuit of the audio frequency amplifier tube with the outputcircuits of both of said demodulators, a loudspeaker coupled with theplate circuit of said audio amplifier tube, a first and a second volumecontrol device each coupled with the plate circuit of a particular oneof the high frequency amplifier tubes and each having connection withthe grid of the respective tube for adjusting the grid bias voltage ofthe tube according to the magnitude of the current supplied to theassociated demodulator, and a master volume control device coupled withthe plate circuit of the low frequency amplifier tube and havingconnection with the grid of each of the high frequency amplifier tubesfor adjusting the grid bias voltage of those tubes according to themagnitude of the current supplied to the loud-speaker.

6. Receiving apparatus for communication systems for railway trainshaving a periodic signaling current which flows partly in the traiiicrails and partly in a line wire paralleling the rails comprising, afirst and a second receiver on a train coupled with the traffic railsand with said line wire respectively, said first receiver mounted on thetrain for effective inductive relationship to the rails andsubstantially non-inductive relationship to the line wire and saidsecond receiver mounted for effective inductive relationship Vto theline wire and substantially non-iiiductive relationship to the rails, afirst and a second amplifier, means for connecting the input circuits ofsaid amplifiers with said rst and second receiver respectively, a firstand a second detector, means for connecting the input circuits of saiddetectors with the output circuits of saidtiirstand second amplifiersrespectively, means*l coupled with the output circuits of both 'of saiddetectors Vfor combining the currents picked up from-the traffic railsand from said line wire after detection, and signaling means controlledrby the last mentioned means responsive -to such signaling current.

7. In railway train communication systems wherein a modulated carriercurrent flows partly in each of two channels one of which includes thetraflic rails and the other of which includes a line conductor extendingalong the track receiving apparatus comprising, a first inductor havingportions mounted on a train directly over each of the rails forinductively receiving energy due to such current flowing in said onechannel, a second inductor mounted relatively high on the train ininductive relation with said conductor for inductively receiving energydue to such current'flowing in said other channel, train carriedamplifier-demodulator means having connections with said first andsecond inductors for receiving :and combining the `energy picked up fromboth of said channels, said amplier-demodulator means operative tocombine the energy picked up from the two channels after demodulation toavoid loss in effectiveness due to phase difference of the currentsflowing in said channels, and signaling means connected With the Outputof said amplier-demodulator means responsive to such combined energy.

8. In 'railway train communication systems wherein a modulated carriercurrent oWs partly in each of two channels one of which includes thetraffic rails and the other of which includes a line conductor receivingapparatus comprising, a rst and a second inductor mounted on a train forinductive relationship with the conductorand the traic railsrespectively, said rst inductor mounted relatively high and in avertical plane on the train and said second inductor mounted on thetrain in a horizontal plane with portion over each rail,amplier-domodulator means on the train having connections with saidinductors for combining the energy picked up from said channels, saidamplier-demodulator means operative to combine the energy afterdemodulation to avoid loss in effectiveness due to phase difference inthe currents flowing in said channels, means connected with the outputof said amplifier-demodulator means for automatically adjusting saidamplier-demodulator to give preference to one or the other of saidchannels, and signaling means connected with the output of saidamplifer-demodulator means responsive to the combined energy.

WILLARD P. PLACE.

